WebGL actually requires the use of index buffers, otherwise the
call is invalid, therefore explicitely request index buffers in
the according VertexAttributeOORTest tests.

In addition, assert on the indices when the glDrawElements call is
captured.

With that we can enable VertexAttributeOORTest.*

Bug: angleproject:6125
Change-Id: Id3855c78d4c5fcab5599f19dd74ce745d059fb1c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2999523
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>